cron

README

cron

A cron library for Go. See the godoc.

Usage

Callers may register Funcs to be invoked on a given schedule. Cron will run them in their own goroutines. A name must be provided.

c := cron.New()
c.AddFunc("0 5 * * * *",  func() { fmt.Println("Every 5 minutes") }, "Often")
c.AddFunc("@hourly",      func() { fmt.Println("Every hour") }, "Frequent")
c.AddFunc("@every 1h30m", func() { fmt.Println("Every hour thirty") }, "Less Frequent")
c.Start()
..
// Funcs are invoked in their own goroutine, asynchronously.
...
// Funcs may also be added to a running Cron
c.AddFunc("@daily", func() { fmt.Println("Every day") }, "My Job")
..
// Inspect the cron job entries' next and previous run times.
inspect(c.Entries())
..
// Remove an entry from the cron by name.
c.RemoveJob("My Job")
..
c.Stop()  // Stop the scheduler (does not stop any jobs already running).

CRON Expression

This section describes the specific format accepted by this cron. Some snippets are taken from the wikipedia article.

Format

A cron expression represents a set of times, using 6 space-separated fields.

Field name	Mandatory?	Allowed values	Allowed special characters
Seconds	Yes	0-59	* / , -
Minutes	Yes	0-59	* / , -
Hours	Yes	0-23	* / , -
Day of month	Yes	1-31	* / , - ?
Month	Yes	1-12 or JAN-DEC	* / , -
Day of week	Yes	0-6 or SUN-SAT	* / , - ?

Note: Month and Day-of-week field values are case insensitive. "SUN", "Sun", and "sun" are equally accepted.

Special Characters

Asterisk ( * )

The asterisk indicates that the cron expression will match for all values of the field; e.g., using an asterisk in the 5th field (month) would indicate every month.

Slash ( / )

Slashes are used to describe increments of ranges. For example 3-59/15 in the 1st field (minutes) would indicate the 3rd minute of the hour and every 15 minutes thereafter. The form "*/..." is equivalent to the form "first-last/...", that is, an increment over the largest possible range of the field. The form "N/..." is accepted as meaning "N-MAX/...", that is, starting at N, use the increment until the end of that specific range. It does not wrap around.

Comma ( , )

Commas are used to separate items of a list. For example, using "MON,WED,FRI" in the 5th field (day of week) would mean Mondays, Wednesdays and Fridays.

Hyphen ( - )

Hyphens are used to define ranges. For example, 9-17 would indicate every hour between 9am and 5pm inclusive.

Question mark ( ? )

Question mark may be used instead of '*' for leaving either day-of-month or day-of-week blank.

Predefined schedules

You may use one of several pre-defined schedules in place of a cron expression.

Entry	Description	Equivalent To
@yearly (or @annually)	Run once a year, midnight, Jan. 1st	0 0 0 1 1 *
@monthly	Run once a month, midnight, first of month	0 0 0 1 * *
@weekly	Run once a week, midnight on Sunday	0 0 0 * * 0
@daily (or @midnight)	Run once a day, midnight	0 0 0 * * *
@hourly	Run once an hour, beginning of hour	0 0 * * * *

Intervals

You may also schedule a job to execute at fixed intervals. This is supported by formatting the cron spec like this:

@every <duration>

where <duration> is a string accepted by time.ParseDuration.

For example, @every 1h30m10s would indicate a schedule that activates every 1 hour, 30 minutes, 10 seconds.

Note: The interval does not take the job runtime into account. For example, if a job takes 3 minutes to run, and it is scheduled to run every 5 minutes, it will have only 2 minutes of idle time between each run.

Time zones

All interpretation and scheduling is done in the machine's local time zone (as provided by the Go time package).

Be aware that jobs scheduled during daylight-savings leap-ahead transitions will not be run!

Thread safety

Since the Cron service runs concurrently with the calling code, some amount of care must be taken to ensure proper synchronization.

All cron methods are designed to be correctly synchronized as long as the caller ensures that invocations have a clear happens-before ordering between them.

Implementation

Cron entries are stored in an array, sorted by their next activation time. Cron sleeps until the next job is due to be run.

Upon waking:

• it runs each entry that is active on that second
• it calculates the next run times for the jobs that were run
• it re-sorts the array of entries by next activation time.
• it goes to sleep until the soonest job.

Documentation

Overview

This library implements a cron spec parser and runner. See the README for more details.

Index

• type ConstantDelaySchedule
  • func Every(duration time.Duration) ConstantDelaySchedule
  • func (schedule ConstantDelaySchedule) Next(t time.Time) time.Time
• type Cron
  • func New(opts ...Option) *Cron
  • func (c *Cron) AddFunc(spec string, cmd func(), name string, tz *time.Location)
  • func (c *Cron) AddJob(spec string, cmd Job, name string, tz *time.Location)
  • func (c *Cron) Entries() []Entry
  • func (c *Cron) QueuingJobs() int
  • func (c *Cron) RemoveJob(name string)
  • func (c *Cron) RunningJobs() int
  • func (c *Cron) Schedule(schedule Schedule, cmd Job, name string)
  • func (c *Cron) Start()
  • func (c *Cron) Stop()
• type Entry
• type FuncJob
  • func (f FuncJob) Run()
• type Job
• type Option
  • func WithMaxWorkers(maxWorkers int) Option
• type Schedule
  • func Parse(spec string, timezone *time.Location) Schedule
• type SpecSchedule
  • func (s *SpecSchedule) Next(t time.Time) time.Time

Types

type ConstantDelaySchedule

type ConstantDelaySchedule struct {
	Delay time.Duration
}

ConstantDelaySchedule represents a simple recurring duty cycle, e.g. "Every 5 minutes". It does not support jobs more frequent than once a second.

func Every

func Every(duration time.Duration) ConstantDelaySchedule

Every returns a crontab Schedule that activates once every duration. Delays of less than a second are not supported (will panic). Any fields less than a Second are truncated.

func (ConstantDelaySchedule) Next

func (schedule ConstantDelaySchedule) Next(t time.Time) time.Time

Next returns the next time this should be run. This rounds so that the next activation time will be on the second.

type Cron

type Cron struct {
	// contains filtered or unexported fields
}

Cron keeps track of any number of entries, invoking the associated func as specified by the schedule. It may be started, stopped, and the entries may be inspected while running.

func New

func New(opts ...Option) *Cron

New returns a new Cron job runner.

func (*Cron) AddFunc

func (c *Cron) AddFunc(spec string, cmd func(), name string, tz *time.Location)

AddFunc adds a func to the Cron to be run on the given schedule.

func (*Cron) AddJob

func (c *Cron) AddJob(spec string, cmd Job, name string, tz *time.Location)

AddFunc adds a Job to the Cron to be run on the given schedule.

func (*Cron) Entries

func (c *Cron) Entries() []Entry

Entries returns a snapshot of the cron entries.

func (*Cron) QueuingJobs

func (c *Cron) QueuingJobs() int

func (*Cron) RemoveJob

func (c *Cron) RemoveJob(name string)

RemoveJob removes a Job from the Cron based on name.

func (*Cron) RunningJobs

func (c *Cron) RunningJobs() int

func (*Cron) Schedule

func (c *Cron) Schedule(schedule Schedule, cmd Job, name string)

Schedule adds a Job to the Cron to be run on the given schedule.

func (*Cron) Start

func (c *Cron) Start()

Start the cron scheduler in its own go-routine.

func (*Cron) Stop

func (c *Cron) Stop()

Stop the cron scheduler.

type Entry

type Entry struct {
	// The schedule on which this job should be run.
	Schedule Schedule

	// The next time the job will run. This is the zero time if Cron has not been
	// started or this entry's schedule is unsatisfiable
	NextScheduleTime time.Time

	// The Job to run.
	Job Job

	// Unique name to identify the Entry so as to be able to remove it later.
	Name string
}

Entry consists of a schedule and the func to execute on that schedule.

type FuncJob

type FuncJob func()

FuncJob A wrapper that turns a func() into a cron.Job

func (FuncJob) Run

func (f FuncJob) Run()

type Job

type Job interface {
	Run()
}

Job is an interface for submitted cron jobs.

type Option

type Option func(*Cron)

func WithMaxWorkers

func WithMaxWorkers(maxWorkers int) Option

type Schedule

type Schedule interface {
	// Return the next activation time, later than the given time.
	// Next is invoked initially, and then each time the job is run.
	Next(time.Time) time.Time
}

The Schedule describes a job's duty cycle.

func Parse

func Parse(spec string, timezone *time.Location) Schedule

Parse returns a new crontab schedule representing the given spec. It panics with a descriptive error if the spec is not valid.

It accepts

• Full crontab specs, e.g. "* * * * * ?"
• Descriptors, e.g. "@midnight", "@every 1h30m"

type SpecSchedule

type SpecSchedule struct {
	Second, Minute, Hour, Dom, Month, Dow uint64
	TimeZone                              *time.Location
}

SpecSchedule specifies a duty cycle (to the second granularity), based on a traditional crontab specification. It is computed initially and stored as bit sets.

func (*SpecSchedule) Next

func (s *SpecSchedule) Next(t time.Time) time.Time

Next returns the next time this schedule is activated, greater than the given time. If no time can be found to satisfy the schedule, return the zero time.